A hydrogen engine using a recirculating (recirculated) working medium has been proposed, in which hydrogen, oxygen, and gaseous argon composed of a monoatomic gas serving as a working medium are supplied to a combustion chamber to combust the hydrogen, and the working medium included in an exhaust gas discharged from the combustion chamber is recirculated to the combustion chamber through a recirculating passage (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 11-93681, Claim 1, paragraphs 0021-0029, and FIG. 1). An argon gas is an inert gas having a very high specific heat ratio. Therefore, the conventional hydrogen engine described above can be operated with greater thermal efficiency, as compared to an engine using a working medium whose specific heat ratio is low. The exhaust gas of the above hydrogen engine contains H2O (steam) and the argon gas. Accordingly, the engine separates/eliminates (or removes) the H2O from the exhaust gas, and resupplies to the combustion chamber the gas from which the H2O has been eliminated.
However, the exhaust gas of the engine described above may contain reaction product generated in the combustion chamber other than the H2O and the argon gas. Examples of such reaction products are carbon dioxide (CO2), nitrogen oxide (NOx), hydrocarbon (HC), and the like. For instance, carbon dioxide is produced when engine lubricant oil is burnt (or chemically altered) in the combustion chamber. More specifically, when a part of the engine lubricant oil staying on a cylinder liner is burnt or when the engine lubricant oil which leaks through oil seal sections of intake valves or of exhaust valves into the combustion chamber is burnt, carbon dioxide is inevitably contained in the exhaust gas.
Incidentally, carbon dioxide is composed of three atoms. The products (the nitrogen oxide, the hydrocarbon, etc.), including the carbon dioxide, produced in the combustion chamber are gases, each of which is composed of two or more atoms. Hereinafter, for convenience sake, the gas composed of two or more atoms is referred to as a “plural atoms gas”. A specific heat ratio of the plural atoms gas is smaller than the specific heat ratio of the monoatomic gas. Therefore, the thermal efficiency of the engine becomes lower as “a concentration of the products formed in the combustion chamber” in the recirculating gas becomes higher.